This invention relates to a sensor for sensing the frequency of fluid pressure fluctuations, and more particularly, to a piezoelectric sensor for a vortex-shedding flowmeter.
Certain bluff bodies, when placed in fluid flow paths, shed vortices in a stable vortex formation known as a von Karman vortex street. With preferred bodies, such as in U.S. Pat. No. 4,350,047 issued on Sept. 21, 1982, the spacing of the vortices of the von Karman vortex street is sufficiently constant over desirable ranges of fluid velocities that vortex frequency can be considered proportional to fluid velocity. Vortex frequency can be measured by sensing the frequency of fluid pressure fluctuation at a fixed point in the street, since the pressure at the fixed point fluctuates according to the presence or absence of a vortex at the point.
Piezoelectric material is a desirable material as a pressure sensor. The material operates without external power supply, and responds well to applied pressure fluctuation, with an electrical signal which can be amplified for electronic processing. As a result, piezoelectric material would seem a good choice for the sensing of fluid pressure fluctuation frequency in a von Karman vortex street. Recognition of this desirability was evidenced long ago in U.S. Pat. Nos. 2,809,520; 3,116,639 and 3,218,852.
One particular vortex-shedding flowmeter pressure sensor that is known to have been offered commercially does employ piezoelectric material. However, that sensor utilizes a number of parts and requires the filling of a chamber with oil. As a result, assembly may be complex. Moreover, the piezoelectric disc employed in that sensor operates in a flexure mode, i.e., the center of the disc moves relative to the disc edges such that the disc is flexed into varying degrees of curvature. It is believed that this mode of operation requires relatively wide deflection of the piezoelectric disc for signal generation and would also entail wide deflection of the diaphragms of the sensor, such that diaphragm fatigue and rupture are a possible concern. Thus, that particular piezoelectric sensor is not wholly advantageous. Simplicity and ruggedness remain in the art as unachieved objectives.
In sum, piezoelectric materials have been long known to be desirable for vortex-shedding flow meter sensors, but the art to date has failed to discover a simplified, rugged piezoelectric sensor suitable for commercial offering as a fluid pressure fluctuation frequency sensor. As a result, many commercial offerings of such sensors continue to be of the hot wire and other non-piezoelectric types.